1. Field of the Invention
This invention relates generally to the field of diagnostic equipment for data communications networks. More particularly, this invention relates to a method and apparatus for performing diagnostics on an access circuit such as a local loop in an Integrated Services Digital Network (ISDN). The method and apparatus provides for an initial determination of B channel error rate without interruptive or disruptive testing. The method and apparatus further provides such initial determination without benefit of knowledge of the protocol of the B channel activity.
2. Background of the Invention
Although the present invention is applicable to diagnostics of ISDN access circuits in general, including those which may include long distance leased lines, it will be described in terms of a simple local loop access circuit for simplicity. Those skilled in the art will recognize that the invention is applicable to primary rate as well as basic rate access to ISDN as well as other rates which may evolve. The present invention is therefore not to be limited to the basic rate example used for illustrative purposes. For purposes of this document, the term "access circuit" is intended to mean the circuit between the user equipment and the ISDN switch.
In basic rate access ISDN, two Bearer channels (B channels) are used to provide 64 KBPS data or voice transmission. According to most standards, this transmission is protocol independent so that it serves to provide a 64 KBPS conduit for user data. In addition to the B channels, a signaling or D channel is provided at 16 KBPS. One of the D channel's primary function is to carry signaling information for the two 64 KBPS B channels. However, due to the bursty nature of the signaling information on the D channel, it is widely anticipated that this channel may also be utilized to carry other types of information when no B channel signaling is in progress. Examples of the types of information anticipated to be carried by the D channel include remote utility meter reading, remote monitoring of alarm systems, and relatively low bit rate terminal data.
A B channel of an ISDN may be either circuit switched or packet switched while the D channel is used to accommodate packet switched data having a packet format very similar to X.25 packets. It is well known that logical channel multiplexing can be accomplished to accommodate many separate logical channels within a single D channel at each node of the network.
Since in most instances it is desirable for user data which is being transmitted over the B channels to remain largely undisturbed so that user data throughput can be maximized, it is desirable to provide a method and apparatus for providing diagnostics functions to the ISDN which minimize interruption or disruption of this user data. The user data which is transmitted over the B channels is generally of unknown protocol as it is usually the function of an ISDN to provide a pipeline for user data without imposing many requirements of the protocol or format for that data. As such, it is generally the case that passive diagnostics which attempt to isolate and correct errors cannot operate in a listen only mode on the B channels since knowledge of the protocol is generally required to accomplish such.
The present invention provides a novel method and apparatus for addressing the needs outlined above by providing a passive diagnostic tool for providing testing of the access circuit of an ISDN.